Piezoelectric actuators have been known as an application of multi-layer piezoelectric elements that are constituted from a plurality of piezoelectric layers stacked via internal electrode layers (metal layers). Piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type which has such a constitution as piezoelectric layers made of piezoelectric porcelain and internal electrode layers having the form of a sheet are stacked alternately one on another. When the requirements to reduce the operating voltage, reduce the manufacturing cost and reduce the thickness and durability are taken into consideration, fired-at-once type piezoelectric actuators are viewed as more advantageous.
FIG. 13A is a perspective view of a multi-layer piezoelectric element of the prior art. This multi-layer piezoelectric element is constituted from a stack 110 and a pair of external electrodes 104 formed on opposing side faces of the stack 110. The stack 110 is formed by stacking a plurality of piezoelectric layers 101 and internal electrode layers 102 alternately one on another. The internal electrode layers 102 are stacked so as to be exposed on opposing side faces of the stack 110 alternately. The pair of external electrodes 104 are formed on the opposing side faces of the stack 110 and the internal electrode layer 102 is connected in every other layer. The external electrode 104 is generally formed by applying an electrically conductive paste that includes an electrically conductive material such as silver and glass to the side faces of the stack 110 and baking (refer, for example, to Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2000-332312, Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2000-31558, Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2005-174974). Inactive layers 109 are formed on both end faces of the stack 110 in the stacking direction.
FIG. 13B is a partial sectional view explaining the constitution of the piezoelectric layers 101 and the internal electrode layers 102 stacked in the multi-layer piezoelectric element described above. As shown in FIG. 13A and FIG. 13B, in the multi-layer piezoelectric element, the internal electrode layers 102 are not formed over the entire principal surface of the piezoelectric layer 101, but are formed in the so-called partial electrode structure where the internal electrode layer 102 has a surface area smaller than that of the piezoelectric layer 101. The stack has the internal electrode layer 102 between two adjacent piezoelectric layers 101 in the stacking direction, and a peripheral area (portion where the internal electrode layer is not formed) between an edge 102a of the internal electrode layer 102 and a side face 110a of the stack 110.
FIG. 14 is a partially enlarged sectional view showing the detail of a junction between the side face of the stack constituting the multi-layer piezoelectric element and external electrodes. As shown in FIG. 14, the stack 110 is constituted from the piezoelectric layers 101 (1011, 1012, . . . , 101n−1 (n≧2)) and the internal electrode layers 102 (1021, 1022, . . . , 104n−1 (n≧2)) stacked alternately one on another. The stack 110 has peripheral areas 111 ( . . . , 111m, . . . 111m+1, . . . (2≦m≦n−3)) where the internal electrode layer 102 is not formed on the principal surface of the piezoelectric layer 101, the peripheral areas 111 being disposed so as to alternately adjoin the pair of external electrodes 104. In this constitution, the internal electrode layers 102 are exposed on different side faces of the stack 110 alternately in every other layer, and are connected to the pair of external electrodes 104 formed on the opposing side faces of the stack 110 in every other layer.
When the multi-layer piezoelectric element is used as a piezoelectric actuator, lead wires 106 are fastened onto the external electrodes 104 by soldering, and a predetermined voltage is applied across the pair of external electrodes 104 via the lead wires 106 so as to drive the multi-layer piezoelectric element. In recent years, as the multi-layer piezoelectric element becomes smaller and is required to undergo a greater amount of displacement under a higher pressure, there is a demand for a multi-layer piezoelectric element that can be operated continuously over a longer period of time with an electric field of higher intensity applied thereto.
However, in the multi-layer piezoelectric element of the prior art, since the internal electrode layer 102 has the partial electrode structure as described above, applying a voltage across the external electrodes 104, 104 causes displacement only in the portion that is interposed between the two internal electrode layers 102 located above and below the piezoelectric layer 101, namely in the portion (displacement portion) where one of the internal electrode layers 102 overlaps with another internal electrode layer 102. The piezoelectric layer 101 does not undergo displacement in the portion (undisplaceable portion) of the piezoelectric layer 101 where the internal electrode layer 102 is not formed (the portion adjacent to the peripheral portion 111), as shown in FIG. 13B and FIG. 14. For example, the piezoelectric layers 101m−1, 101m located in the stacking direction on both sides of the peripheral area 111m of the internal electrode layer 102m are interposed between the internal electrode layers 102m−1, 102m+1 of the same polarity. As a result, the piezoelectric layers 101m−1, 101m located in the stacking direction on both sides of the peripheral area 111, are not subjected to the electric field, and therefore do not undergo displacement. Moreover, since the two piezoelectric layers 101m−1, 101m located on both sides of the peripheral area 111m in the stacking direction are firmly joined to each other, there is such a problem that the peripheral area 111m restricts the displacement of the entire stack, thereby decreasing the amount of displacement of the element. Similar problems exist also with the other peripheral area such as the peripheral area 111m+2. The peripheral areas 111 provide the functions of ensuring insulation for alternately connecting the internal electrode layers 102 to the pair of external electrodes 104, and increasing the strength of the multi-layer piezoelectric element against breakage by joining the piezoelectric layers 101 with each other without the internal electrode layer 102 interposed therebetween.